Fuel cells have been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. A plurality of fuel cells can be stacked together in series to form a fuel cell stack capable of supplying a desired amount of electricity. The fuel cell stack has been identified as a potential alternative to the traditional internal-combustion engine used in automobiles.
One type of fuel cell is known as a proton exchange membrane (PEM) fuel cell. The PEM fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte membrane therebetween. The anode receives hydrogen gas and the cathode receives oxygen. The hydrogen gas is catalytically disassociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
The fuel cell stack typically receives oxygen for the cathodes from charged air provided by an air compressor. It is known in the art to employ in a fuel cell system a turbo-machine type compressor that has a rapidly rotating rotor that increases the velocity and pressure of fluid moving therethrough. Typical turbo-machine type compressors include centrifugal, radial, axial, mixed flow, and the like. Turbo-machine type compressors are low in cost and weight, and operate with minimal noise. Another category of compressor known for use in fuel cell systems is the “positive displacement” compressor. The positive displacement compressor has at least one rotor in close proximity to another rotor or to a stator. The positive displacement compressor is well known in the art and includes rotary machines such as scroll machines, vane machines and screw machines, roots blowers, and the like.
The typical air compressor operates according to a compressor map of pressure ratio (outlet pressure/inlet pressure) versus mass flow. FIG. 1 is an exemplary compressor map 10 for a turbo-machine type compressor, showing mass flow on the horizontal axis and pressure ratio on the vertical axis. The compressor map 10 includes a series of speed lines 12 that show the relationship between mass flow through the air compressor and the pressure ratio across the air compressor at various compressor speeds. Most air compressors are generally provided with compressor maps 10 and operate according thereto.
The compressor map 10 is bound by a surge line 14 and a choke line 15. When operating under conditions exceeding the surge line 14, the air compressor suffers from a flow reversion caused by excessive back-pressure, The surge line 14 is determined by a number of factors, including the speed or RPM of the compressor, the system back-pressure, altitude and temperature. Excessive back-pressure from the fuel cell system, in particular, may cause a compressor surge event. The surge event may result in an undesirable oscillation of the airflow through the air compressor.
One known system and method that employs surge prevention by electronically mapping the compressor for discharge pressure versus mass airflow is disclosed in U.S. Pat. Application No. 2005/0164057 to Pospichal et al., hereby incorporated herein by reference in its entirety. Another known system and method for surge avoidance that employs a control valve for opening and closing a recirculation valve in response to detection of an incipient surge condition is described in U.S. Patent Application No. 2004/0161647 to Rainville et al., hereby incorporated herein by reference in its entirety.
A surge control distance (SCD) 16 is a known control parameter used to avoid the surge event when operating the air compressor in the fuel cell system. The SCD 16 is a desired difference between a current operating condition of the air compressor and a minimum stable operating condition dictated by the surge line 14. The SCD 16 for a standard air compressor is generally statistically based on a relevant sample of like air compressors. The SCD 16 accounts for part-to-part variation in the air compressor, variation in fuel cell system sensors, and changes to the air compressor tolerance due to wear with use. It is desirable to minimize the SCD 16, however, in order to maximize the efficiency of the air compressor and minimize the use of bypass airflow typically employed to correct surge events.
There is a continuing need for a system and method of maximizing the efficiency of the air compressor in the fuel cell system, minimizing the quantity of compressor bypass air, and avoiding surge events.